The anus is the external opening through which feces are expelled from the body. Just inside the anus are a number of small glands. If one of these glands become blocked, an abscess—an infected cavity—may form. An anal abscess is usually treated by surgical drainage, although some drain spontaneously. About 50% of these abscesses may develop into a fistula, in which a small tunnel connects the infected gland inside the anus to an opening on the skin around the anus.
Most fistulas result from an anal abscess. A small number of fistulas may less frequently be caused by other processes such as Crohn's disease, sexually transmitted diseases, trauma, tuberculosis, cancer, or diverticulitis.
The following may be symptoms or signs of an anal fistula:
You should see your physician if you notice any of these symptoms.